Method for fabricating preplated nickel/palladium and tin leadframes

ABSTRACT

A method for fabricating a leadframe structure comprising a chip mount pad and a plurality of lead segments, each having a first end near the mount pad and a second end remote from said mount pad. The structure is formed from a sheet-like starting material. In a first plating system, the leadframe is plated with a layer of nickel. Next, the second segment ends are selectively masked and a layer of palladium is selectively plated on the nickel layer on the exposed chip pad and first segments ends in a thickness suitable for wire bonding attachment. In a second plating system, the chip pad and first segment ends are selectively masked and a pure tin layer is selectively plated on the nickel layer on the exposed second segment ends in a thickness suitable for parts attachment.

FIELD OF THE INVENTION

[0001] The present invention is related in general to the field ofsemiconductor devices and processes and more specifically to thematerials and fabrication of leadframes for integrated circuit devices.

DESCRIPTION OF THE RELATED ART

[0002] The leadframe for semiconductor devices was invented (U.S. Pat.No. 0 3,716,764 and No. 4,034,027) to serve several needs ofsemiconductor devices and their operation simultaneously: First of all,the leadframe provides a stable support pad for firmly positioning thesemiconductor chip, usually an integrated circuit (IC) chip. Since theleadframe including the pads is made of electrically conductivematerial, the pad may be biased, when needed, to any electricalpotential required by the network involving the semiconductor device,especially the ground potential.

[0003] Secondly, the leadframe offers a plurality of conductive segmentsto bring various electrical conductors into close proximity of the chip.The remaining gap between the (“inner”) tip of the segments and theconductor pads on the IC surface are typically bridged by thin metallicwires, individually bonded to the IC contact pads and the leadframesegments. Obviously, the technique of wire bonding implies that reliablewelds can be formed at the (inner) segment tips.

[0004] Thirdly, the ends of the lead segment remote from the IC chip(“outer” tips) need to be electrically and mechanically connected to“other parts” or the “outside world”, for instance to assembly printedcircuit boards. In the overwhelming majority of electronic applications,this attachment is performed by soldering. Obviously, the technique ofsoldering implies that reliable wetting and solder contact can beperformed at the (outer) segment tips.

[0005] It has been common practice to manufacture single pieceleadframes from thin (about 120 to 250 μm) sheets of metal. For reasonsof easy manufacturing, the commonly selected starting metals are copper,copper alloys, iron-nickel alloys (for instance the so-called “Alloy42”), and invar. The desired shape of the leadframe is etched or stampedfrom the original sheet. In this manner, an individual segment of theleadframe takes the form of a thin metallic strip with its particulargeometric shape determined by the design. For most purposes, the lengthof a typical segment is considerably longer than its width.

[0006] In the European patent #0 335 608 B1, issued Jun. 14, 1995(Abbott, “Leadframe with Reduced Corrosion”), U.S. Pat. No. 6,194,777,issued Feb. 27, 2001 (Abbott, “Leadframes with Selective PalladiumPlating”), and No. 6,246,446, issued Jun. 12, 2001 (Abbott, “Leadframewith Reduced Corrosion”), a palladium-plated leadframe is introducedwhich is not subject to corrosion due to galvanic potential forcesaiding the migration of the base metal ions to the top surface wherethey will form corrosion products. The patent describes a sequence oflayers consisting of nickel (over the base metal), palladium/nickelalloy, nickel, and palladium (outermost). This technology has beenwidely accepted by the semiconductor industry.

[0007] After assembly on the leadframe, most ICs are encapsulated,commonly by plastic material in a molding process. It is essential thatthe molding compound, usually an epoxy-based thermoset compound, hasgood adhesion to the leadframe and the device parts it encapsulates.Palladium, described above as the outermost layer of the leadframe,offers excellent adhesion to molding compounds.

[0008] Nickel/palladium plated leadframes are used because of their lowtotal cost of ownership, primarily a result of eliminating post-moldsolder plating. However, some customers, for instance automotivemanufacturers and telephone central switching offices, require solderplated external leads, typically because of burn-in, accelerated testingor environmental conditions.

[0009] If solder dipping is used after molding, the palladium willdissolve into the solder and the nickel is then solderable. However,solder dipping is not practical for devices with fine-pitch leadframesbecause of solder bridging.

[0010] The price of palladium climbed in the last decade toapproximately twice the gold price. Cost reduction pressures insemiconductor manufacturing have initiated efforts to reduce thethickness of the palladium layers employed to about one third of itsprevious thickness. At this thinness, palladium does not preventoxidation of the underlying nickel which will inhibit its solderability.

[0011] In U.S. patent applications Ser. No. 60/138,070, filed on Jun. 8,1999 (Abbott, “Palladium-Spot Leadframes for Solder Plated SemiconductorDevices and Method of Fabrication”), and No. 60/214,314, filed on Jun.27, 2000 (Abbott, “semiconductor Leadframes Plated with Lead-Free Solderand Minimum Palladium”), to which the present invention is related, afabrication process for palladium layers of reduced thickness isdescribed. There is, however, a problem for pre-plated leadframes in theselection of a tin-based solder without the risk of tin dendrite growth.Further, there is a severe discrepancy between the high speed andthroughput of nickel and palladium plating for the desired thicknesses,and the roughly 20× slower tin plating for the desired thickness.

[0012] An urgent need has therefore arisen for a low-cost, reliable massproduction method for a leadframe combining the advantages of palladiumwith its bondability and adhesion capability to molding compounds, andthe application of pre-plated tin solder. The palladium layer shouldhave reduced thickness. The leadframe and its method of fabricationshould be flexible enough to be applied for different semiconductorproduct families and a wide spectrum of design and assembly variations,and should achieve improvements toward the goals of improved processyields and device reliability. Preferably, these innovations should beaccomplished using the installed equipment base so that no investment innew manufacturing machines is needed.

SUMMARY OF THE INVENTION

[0013] The present invention describes a method for fabricating aleadframe structure comprising a chip mount pad and a plurality of leadsegments, each having a first end near the mount pad and a second endremote from said mount pad. The structure is formed from a sheet-likestarting material. In a first plating system, the leadframe is platedwith a layer of nickel. Next, the second segment ends are selectivelymasked and a layer of palladium is selectively plated on the nickellayer on the exposed chip pad and first segments ends in a thicknesssuitable for wire bonding attachment. In a second plating system, thechip pad and first segment ends are selectively masked and a pure tinlayer is selectively plated on the nickel layer on the exposed secondsegment ends in a thickness suitable for parts attachment.

[0014] The present invention is related to high density ICs, especiallythose having high numbers of inputs/outputs, or contact pads, and alsoto devices in packages requiring surface mount in printed circuit boardassembly. These ICs can be found in many semiconductor device familiessuch as standard linear and logic products, digital signal processors,microprocessors, digital and analog devices, and both large and smallarea chip categories. The invention represents a significant costreduction and enhances environmental protection and assembly flexibilityof semiconductor packages, especially the plastic molded packages,compared to the conventional copper-based post-mold-plated leadframes.

[0015] It is an aspect of the present invention to provide a technologyfor enabling solder package leads with pre-plated pure tin, whilemaintaining a palladium layer in the localized areas intended for wirebonding with its significant cost advantage over the traditional silverspot plated inner leads.

[0016] Another aspect of the invention is to provide the tin as a matte,coarse grain, low carbon content deposit of relatively high thickness(about 4 to 6 μm). The tin annealing is a by-product of the moldingcompound curing after encapsulation. Tin whiskers on the outside of thepackage are suppressed by the nickel.

[0017] Another aspect of the invention is to reach these goals with alow-cost manufacturing method without the cost of equipment changes andnew capital investment, by using the installed fabrication equipmentbase.

[0018] The invention utilizes a first, wheel-based plating system todeposit the nickel and palladium layers, and a second, flood cell systemto deposit the tin layer.

[0019] Another aspect of the invention is to produce leadframes so thatestablished wire bonding processes can continue unchanged, and thatestablished board attachment process can continue unchanged.

[0020] Another aspect of the invention is to eliminate silver and thecyanide solution used for its deposition from the leadframemanufacturing process flow, resulting in less costly waste treatment.

[0021] Another aspect of the invention is to introduce a palladium spotplating technology with provides loose tolerance for the spotboundaries, thus simplifying leadframe manufacturing and loweringfabrication cost.

[0022] These aspects have been achieved by the teachings of theinvention concerning deposition and masking methods suitable for massproduction. Various modifications of leadframe preparations have beensuccessfully employed.

[0023] In the preferred embodiment of the invention, a plated layer ofnickel is fully covering the leadframe base material. A plated layer ofpure tin is plated onto the nickel layer so that it covers selectivelyleadframe areas intended for parts attachment, especially boardassembly. A layer of palladium is then plated onto the nickel layer sothat it covers selectively the leadframe areas intended for bonding wireattachment.

[0024] Leadframes prepared according to the invention can besuccessfully used in surface mount technologies based on bending thepackage lead segments.

[0025] The technical advances represented by the invention, as well asthe aspects thereof, will become apparent from the following descriptionof the preferred embodiments of the invention, when considered inconjunction with the accompanying drawings and the novel features setforth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is a schematic and simplified cross sectional view of aleadframe with base metal and first plated metal.

[0027]FIG. 2 is a schematic and simplified cross sectional view of aleadframe after the plating step for wire bonding enhancement accordingto the invention.

[0028]FIG. 3 is a schematic and simplified cross sectional view of aleadframe after the plating step for pure tin according to theinvention.

[0029]FIG. 4 is a block diagram of the process flow through the firstplating system and the second plating system according to the invention.

[0030]FIG. 5 is a schematic and simplified perspective view of the tinflood plating cell used in the second plating system according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention is related to U.S. patent applications Ser.No. 0 60/214,314, filed on Jun. 27, 2000 (Abbott, “SemiconductorLeadframes Plated with Lead-free Solder and Minimum Palladium”) and#TI-33737, submitted on Feb 1,2002 (Abbott et al., “SemiconductorLeadframes Plated with Thick Nickel, Minimum Palladium, and Pure Tin”),which are herewith incorporated by reference. The present invention isfurther related to U.S. patent application Ser. No. 60/138,070, filed onJun. 8, 1999 (Abbott, “Palladium-Spot Leadframes for Solder PlatedSemiconductor Devices and Method of Fabrication”).

[0032] The present invention is related to the composition andsequential construction of semiconductor leadframes and to the assemblyof semiconductor chips on leadframes. This assembly includes wirebonding interconnection, chip encapsulation, and the environmentallyfriendly process of reliable attachment of the devices to substratesusing lead-free solder.

[0033] The invention reduces the cost of leadframes while the leadframefunctions are maximized. The invention best applies to any leadframe andany substrate used in semiconductor technology which exhibit thefollowing design features: Usually, a chip mount pad for support of theIC chip surrounded by lead segments, each having a first end inproximity of the chip pad, and a second end remote from the chip pad.The invention thus applies to semiconductor package types such as PDIPs,SOICs, QFPs, SSOPs, TQFPs, TSSOPs and TVSOPs.

[0034] As defined herein, the starting material of the leadframe iscalled the “base metal”, indicating the type of metal. Consequently, theterm “base metal” is not to be construed in an electrochemical sense (asin opposition to ‘noble metal’) or in a structural sense. The base metalof leadframes is typically copper or copper alloys. Other choicescomprise brass, aluminum, iron-nickel alloys (“Alloy 42”), and invar.

[0035] Leadframe segments have to satisfy five needs in semiconductorassembly:

[0036] 1) Leadframes have to comprise segment ends remote from the chipmount pad (“outer segments”) for solder attachment to other parts;

[0037] 2) leadframes have to comprise segment ends near the chip mountpad (“inner segments”) for bond attachments to wire interconnections;

[0038] 3) leadframes have to comprise outer segments ductile for formingand bending the segments;

[0039] 4) leadframe surfaces have to comprise adhesion to moldingcompounds; and

[0040] 5) leadframe segments have to comprise insensitivity tocorrosion.

[0041] According to the teachings of this invention, Need 1) issatisfied by depositing a layer of nickel, fully covering the leadframebase metal, and then selectively preplating a layer of pure tin onto thenickel layer only onto those leadframe areas which are intended forparts attachment.

[0042] The invention satisfies Need 2) by first plating the nickellayer, fully covering the leadframe base metal as outlined above, andthen plating a thin layer of palladium (or, if desired, of silver) ontothe nickel layer, selectively covering areas of the leadframe which areintended for bonding wire attachment (and chip attachment). Forpalladium, a thin layer is sufficient for reliable bonding wireattachment (stitch bonds, ball bonds, or wedge bonds).

[0043] The invention satisfies Need 3) by the selection of thickness andstructure of the nickel layer employed to fulfill need 1). Thickness anddeposition method of the nickel layer have to be selected such that thelayer insures ductility and enables the bending and forming of the outerlead segments.

[0044] The invention satisfies Need 4) by the choice of the noble metallayer employed to fulfill need 2); a practical selection is palladiumwith its excellent adhesion to thermoset molding compounds and otherencapsulation materials.

[0045] The invention satisfies Need 5) by the sequence of layersdeposited over the copper base: Nickel and pure tin.

[0046]FIG. 1 is a schematic and simplified cross section of a leadframeportion, generally designated 100, and shows the chip mount pad 101 anda plurality of lead segments 102. The leadframe is made of a base metal103 fully covered with a plated layer 104. The base metal usually iscopper or copper alloy, but may also be aluminum, brass, an iron-nickelalloy, or invar. The copper or copper alloy base sheet 103 has apreferred thickness in the range from 100 to 300 μm; thinner sheets arepossible. The ductility in this thickness range provides the 5 to 15%elongation needed in the segment bending and forming operation. Theleadframe is stamped or etched from the starting metal sheet. The platednickel layer has a preferred thickness is the range from about 0.2 to3.0 μm.

[0047] In the plating process, the stamped or etched leadframe is firstimmersed in an alkaline preclean solution at 20 to 90° C. for fewseconds up to 3 minutes. Both alkaline soak cleaning and alkalineelectrocleaning are employed. Oils, grease, soil, dirt and othercontamination are thereby removed.

[0048] After rinsing, the leadframe is next immersed in an acidactivation bath at room temperature for few seconds up to 5 minutes. Thebath consists of a solution of sulfuric acid, hydrochloric acid, orother acid solution, preferably at about 30 to 60 g/l concentration.This solution removes copper oxide and leaves the metallic coppersurface in an activated state, ready to accept the deposition ofmetallic nickel.

[0049] Next, the leadframe is immersed in a first nickel platingsolution to receive the deposition onto the copper base material of anickel strike in the thickness range of about 0.02 to 0.13 μm. Thisfirst nickel layer fully encases the copper base metal and thus keepsthe subsequent main nickel bath free from copper and copper compounds.

[0050] Next, the leadframe is immersed in a second nickel platingsolution to receive the deposition onto the first nickel layer of anadditional nickel layer in the thickness range of about 0.45 to 2.0 μm.The total thickness range of layer 104 is approximately 0.5 to 3.0 μm.This nickel layer has to be ductile for the leadframe segment bendingand forming process. Further, the nickel surface has to be wettable inthe soldering process, so that solder alloys or conductive adhesives canbe used successfully.

[0051] The schematic cross section of a leadframe in FIG. 2 depicts thethin palladium layer 205, plated on the area of chip pad 201 and thefirst ends 203 of segment 202 near the chip pad. The deposited layer 205comprises an electroplated palladium layer in the thickness range ofabout 20 to 60 nm. Another choice as noble metal would be silver orrhodium. The palladium thickness could possibly reduced to about 10 and30 nm. The area portion 203 of the plurality of lead segments isdetermined by the technical requirement of the intended wire bondingattachment. In the schematic example of FIG. 2, layer 205 terminates atboundary 205 a. It is an advantage of the invention that boundary 205 amay have loose tolerances.

[0052] In this thickness range, palladium is suitable for all wirebonding attachments (stitch bonds, ball bonds, and wedge bonds) andretains its excellent adhesion to thermoplastic molding compounds—anattribute crucial for avoiding package delamination and progressivecorrosion. It should further be noted that the surface of the leadframethat is not spot-plated with palladium has on the order of 1 to 5 nm ofpalladium on it. Consequently, the surface has some nickel and somepalladium character to it. After the thermal excursions of the deviceassembly process, the surface should have an adherent layer of nickeloxide that can provide superior molding compound adhesion.

[0053] It is an important aspect of the present invention to deposit thepalladium layer selectively onto the leadframe by using an inexpensivemasking step. The selective characteristic of the palladium depositionis achieved by a temporary masking step, which leaves only thoseleadframe portions exposed which are intended to receive the palladiumlayer.

[0054] There are several methods to selectively deposit metals fromsolution onto a continuous strip. For high volume production ofleadframes, continuous strip or reel-to-reel plating is advantageous andcommon practice. Based on the loose tolerance acceptable for theboundaries of the palladium plating on the inner ends of the leadsegments, the preferred deposition method for the present invention isthe so-called “wheel system”. The process steps are as follows.

[0055] WHEEL SYSTEM

[0056] Material is moved over a large diameter wheel with apertures init to allow solution flow to material;

[0057] apertures define the locations for plating; index pins engage thepilot holes (designated 38 in FIG. 3) in the leadframe;

[0058] backing belt is used to hold material on wheel and mask backsideof material;

[0059] anode is stationary inside wheel.

[0060] Advantages: Fast, material never stops for selective plating; notiming issues; pumps, rectifiers, and drive system are on continuously;low cost because system is mechanically uncomplicated.

[0061] Disadvantages: Loose plating boundaries, poor spot location, andpotential bleedout are not critical issues for the present invention.

[0062] A more precise, but also more costly and slower selective platingtechnique is the step-and-repeat process.

[0063] STEP AND REPEAT

[0064] Leadframe material is stopped in selective plating head;

[0065] rubber mask system clamps on material;

[0066] plating solution is jetted at material;

[0067] current is applied;

[0068] current is shut off;

[0069] solution is shut off;

[0070] head opens;

[0071] material moves.

[0072] Advantages: Very sharp plating spot with excellent edgedefinition; very good spot location capability when used with indexholes, pins and feedback vision system.

[0073] Disadvantages: Slow; material must stop during selective plating;expensive equipment to buy and maintain; timing issues; lots of movingparts.

[0074] The schematic cross section of a leadframe in FIG. 3 depicts arelatively thick pure tin layer 306, which is plated selectively ontothe flood plated layer 104 of nickel over the leadframe base material103. The tin layer has a thickness in the range from about 4.0 to 10.0μm. The tin plated portions cover the areas of the leadframe intendedfor board attach or other parts attachment, specifically the second endsof the lead segments 302, remote from the chip mount pad. The boundaryof the solder plated portion is designated 306 a in FIG. 3. It is anadvantage of the invention that boundary 306 a may have loosetolerances.

[0075] It is of pivotal importance to the present invention that

[0076] the pure tin represents a lead-free solder;

[0077] the tin is deposited as a pre-plated layer, i.e., applied to theleadframe before the start of chip assembly;

[0078] the tin has a reflow temperature of 232° C. so that IC assemblytemperatures up to about 215° C. are tolerable, including wire bondingand package molding;

[0079] the tin is able to dissolve into the solder flux or wave duringdevice board attach, and

[0080] the tin avoids tin whisker growth.

[0081] It is helpful for suppressing whisker growth that the pre-platedtin layer is in a matte, coarse grain, and low carbon contentcomposition. Most important is the fact that the tin layer receives, dueto its pre-plating deposition before the molding encapsulation process,a thorough annealing step during the extended molding compoundpolymerization period (“curing”; commonly at 175° C. for 5 to 6 hr) . Itis a technical advantage of the invention that this pivotal annealingstep is provided without any additional time or cost during the assemblyprocess.

[0082] It is an important aspect of the present invention to deposit thetin layer selectively onto the leadframe using a separate plating systemwith its characteristic speed and an inexpensive, temporary maskingstep, which leaves only those leadframe portions exposed which areintended to receive the solder layer. This tin plating system isdepicted in FIG. 4 in conjunction with the nickel and palladium platingsystem as a complete process flow.

[0083]FIG. 4 depicts the complete process flow according to theinvention, generally designated 400, of plating the base metal of theleadframe with the nickel, palladium, and tin layers in two parts:Plating the nickel and palladium layers in a first plating system 410,coupled to a second plating system 420 including the tin platingstations. The stations of the first plating system 410 summarize thesteps described above:

[0084]411: The input is a leadframe made of stamped or etched from abase metal such as copper;

[0085]412: cleaning of the leadframe, and

[0086]413: activating prepares the base metal for:

[0087]414: plating the nickel layer;

[0088]415: spot plating the palladium layer can be provided by the wheelmethod described above, or the step-and-repeat method;

[0089]416: rinsing and

[0090]417: drying complete the palladium plating phase. The leadframehas now the configuration depicted in FIG. 2 and exits the first platingsystem 410. The average throughput speed of system 410 is about 35ft/min and thus about 20 times faster than the throughput speed of thesecond plating system 420 for the tin plating (order of 2 ft/min).

[0091]421: The input is the nickel and palladium-plated leadframe fromsystem 410; it enters system 420;

[0092]422: wetting; no cleaning of the leadframe is required;

[0093]423: activating;

[0094]424: coating the leadframe with a plating resist;

[0095]425: photoimaging; a wheel is used with apertures defining theexposed areas of the resist; this is a line-of-sight process, devoid ofsealing or bleedout issues. The imaging step is a dry process;consequently, it offers leadframe design opportunities not available fora wheel that must withstand plating solutions. The imaging is performedcontinuously, synchronized with all the other process steps;

[0096]426: developing; removing the resist in areas intended forplating;

[0097]427: tin plating; the tin plating equipment is designed as a floodcell and is schematically illustrated in FIG. 5. The flood cell,generally designated 500, is a simple polypro box with anode baskets 501as soluble tin anodes lined along length L (designated 502) of the cell.The leadframe enters at 503 and exits at 504. The tin plating throughputcan be increased by increasing the length L of cell 500. The residencetime is a function of L and the leadframe speed; higher speed can beobtained by increasing length L. The electrolytic solution enters thecell at 505.

[0098]428: stripping the plating resist;

[0099]429: rinsing; and

[0100]430: drying complete the tin plating phase. The leadframe has nowthe configuration depicted in FIG. 3 and exits the second plating system420 at the

[0101]431: output as a pre-plated nickel/palladium and tin leadframe.

[0102] The selective plating method of the invention offers a number ofadvantages in comparison to the alternative, conventional selectiveplating methods, namely the wheel method and the step-and-repeat methoddescribed above. In the step-and-repeat method, a rubber mask is clampedon the stationary leadframe during the plating process to define theplated area. This process results in excellent spot definition, but alsoin mechanical and timing issues in the plating head, which is short(about 12 inch) and thus causes a line speed for the tin deposit on theorder of 2 ft/min. The step-and-repeat method is thus complicated andcostly. For the wheel system, sealing of the mask to the leadframe isnot as good as in the step-and-repeat method, so spot definition is notas good and tin deposit bleedout occurs. The size of the tin platingcell is limited, restricting the line speed. In both methods, thepotential for tin contamination of the existing palladium spot is high.

[0103] In contrast, in the selective tin plating method of theinvention:

[0104] no tin contamination of the palladium spot occurs, since thepalladium spot is protected by the plating mask;

[0105] high plating throughput is provided by a long tin plating floodcell;

[0106] soluble tin anodes can easily be incorporated into the tin cell,resulting cost savings;

[0107] the tin spot definition is excellent;

[0108] reel-to-reel technology can be applied;

[0109] the plating line is uncomplicated and thus low cost;

[0110] imaging and printing are performed continuously, in-line with theother steps in the tin plating process;

[0111] low cost plating resist can be used, since the plating process isadditive and does not have the requirements of a subtractive etchprocess; and

[0112] alternatively, a printable plating mask could be used thateliminates the need for photoimaging and developing.

[0113] While this invention has been described in reference toillustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. As an example, the designs, cover areas andfabrication methods of the tin layer and of the palladium layer may bemodified to suit specific leadframe or substrate needs. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

I claim:
 1. A method for fabricating a leadframe structure comprising achip mount pad and a plurality of lead segments, each having a first endnear said mount pad and a second end remote from said mount pad,comprising the steps of: forming said structure from a sheet-likestarting material; plating a layer of nickel on said leadframe;selectively masking said second segment ends, thereby leaving said chippad and said first segment ends exposed; selectively plating a layer ofpalladium on said nickel layer on said exposed chip pad and segment endsin a thickness suitable for bonding wire attachment; selectively maskingsaid chip pad and said first segment ends, thereby leaving said secondsegment ends exposed; and selectively plating a layer of tin onto saidnickel layer on said exposed segment ends in a thickness suitable forparts attachment.
 2. The method according to claim 1 wherein saidplating of said nickel layer and said plating of said selectivepalladium layer are performed in a first plating system providing forsaid palladium plating a wheel with apertures defining said selectivelocations.
 3. The method according to claim 1 wherein said plating ofsaid tin layer is performed in a second plating system providingphoto-imagible or printable plating masks.
 4. The method according toclaim 3 wherein said second plating system provides: coating with aplating resist; photoimaging; developing said resist; plating with tin;stripping said resist; rinsing; and drying.
 5. A method for fabricatinga leadframe comprising the steps of: stamping from a sheet-like copperor copper alloy starting material a leadframe having a mount pad for anintegrated circuit chip and a plurality of lead segments having theirfirst end near said mount pad and their second end remote from saidmount pad; in a first plating system, cleaning said leadframe inalkaline soak cleaning and alkaline electrocleaning; activating saidleadframe by immersing said leadframe into an acid solution, therebydissolving any copper oxide; immersing said leadframe into a firstelectrolytic nickel plating solution and depositing a first layer ofnickel onto said copper, thereby fully encasing said copper; immersingsaid leadframe into a second electrolytic nickel plating solution anddepositing a second layer of nickel onto said first nickel layer,thereby adapting said second ends of said lead segments for mechanicalbending and solder attachment; selectively masking said second segmentends thereby leaving, through apertures in said wheel, said chip pad andsaid first segment ends exposed; immersing said leadframe into anelectrolytic palladium plating solution and depositing a layer ofpalladium onto said exposed segment ends in a thickness suitable forbonding wire attachment; in a second plating system, selectively maskingsaid chip pad and said first segment ends, thereby leaving said secondsegment ends exposed, said masking provided by photoresist coating,photoimaging, and resist developing; immersing said leadframe into a tinflood cell plating solution and depositing a layer of tin onto saidexposed second segment ends in a thickness suitable for partsattachment; and stripping said photoresist, rinsing and drying.
 6. Themethod according to claim 5 wherein said first plating system is awheel-based system, and said second plating system is a flood cellsystem.
 7. The method according to claim 5 wherein the process steps areexecuted in sequence without time delays, yet including intermediaterinsing steps.
 8. The method according to claim 5 wherein said acidsolution may be sulfuric acid, hydrochloric acid or any other acid. 9.The method according to claim 5 wherein said photoimaging of saidphotoresist uses a wheel with apertures defining the exposed area ofsaid resist.